This invention relates generally to an apparatus and method for unloading production-inhibiting liquid from a well and more particularly to a gas lift technique for intermittently removing water which limits the flow of natural gas from a subterranean formation.
One way to produce gas from a well is by depletion or volumetric drive. Gas is driven from the well by pressure naturally existing within the formation from which the gas comes. Over time, however, the formation pressure typically declines sufficiently that it cannot overcome the backpressure exerted by liquids which may accumulate in the well adjacent the formation.
In producing gas, water typically condenses and falls out of the upwardly flowing stream of gas being produced out of the well. Over time, this water will accumulate in the well across the formation. This restricts flow out of the formation, and in some cases at least, this can stop further production altogether. Even where some production can still occur, it may be uneconomical to continue it. In some wells, even a relatively small amount of water falling out of the gas production stream can have this detrimental effect.
Where this condition prevents economic production by formation pressure alone, either the well is abandoned or another means of recovering the gas from the formation must be used. The latter is done if the cost of recovery is less than the value of what is recovered.
Techniques which have been used or proposed to try to meet this cost versus value criterion include using soap sticks, "pitting" the well occasionally (blowing the well down in a pit to atmospheric pressure), swabbing, injecting high pressure gas in the formation, lowering the end of a tubing string to the perforations, tapering the tubing string to a smaller inner diameter near the surface to increase rate of flow, optimizing tubing size to balance velocity and friction effects, using plunger lift, waterflooding the formation to augment pressure depletion, insulating and heating the production tubing string to minimize condensation and liquid fallout, and beam lifting. "Stop cocking," wherein the annulus of the well is closed so that pressure increases in response to formation pressure migrating through the accumulated liquid or unaffected portions of the formation until it can drive the liquid down the annulus and up a communicating tubing string, is another known technique.
Whether any of the foregoing is appropriate for a given case depends on the specific well conditions. For example, at least some of the foregoing require a substantial initial investment, and at least some require sufficient formation energy to lift the obstructing liquid.
Another known technique is gas lift, wherein at least a portion of the liquid is collected in a chamber and then a pressurized gas is injected into the chamber from outside the well to lift collected liquid out of the well through a lift tubing string communicating with the chamber. The chamber is defined either by two spaced packers and the intervening length of the well (e.g., a casing or liner) or by a vessel connected to the lift tubing string. The pressurized gas is injected into the chamber either through the annulus or an injection tubing string. Thus, either a single tubing string is used, or two tubing strings are used. The dual tubing string methods we are aware of have the strings either embedded one within the other or externally adjacent to each other but interconnected to communicate between the two tubing strings above where the strings connect to the chamber. These implementations provide relatively limited increased production because they produce only through the relatively small diameter lift tubing or they do not produce during the lift cycle. With regard to the dual string implementations, in particular, the ones we are aware of require relatively complex crossover elements or multiple standing valves. These can increase maintenance problems and cost.
Although the foregoing techniques can be useful in particular applications, there is still the need for an improved apparatus and method for unloading production-inhibiting liquid from a well to have utility at least in part where the previously known techniques may not be suitable. Such an improved technique should preferably be suitable for efficiently unloading at least relatively small amounts of liquid from low pressure formations which are deep enough that it may not be economical to use other production techniques but not so deep that the necessary amount of liquid cannot be economically lifted by gas injected from a source outside the well. Thus, the improved technique should provide an external source of energy to lift liquid out of the well; however, the energy should be confined to avoid injecting high pressure gas into the formation. The improved technique should also provide for maximum production and reduced friction pressure by allowing continuous gas flow up the well annulus. The improved technique also should be capable of use below perforations into the formation to reduce formation hydrostatic backpressure. To reduce cost and to facilitate installation, use and maintenance, the improved technique should also be relatively simple.